1. Field of the Invention
The present invention concerns a new and improved system for the measurement of the gas content of blood of a living being in a region of its body, and particularly for the optical measurement of the oxygen saturation of blood flowing through said region of the body. Systems of this kind are used for example for noninvasively determining the oxygen saturation of blood, i.e. without any instruments to penetrate through the skin or through any other bodily surface, in patients having disturbed oxygen supply as a consequence of sickness or of surgical operations or during the medical examination of athletes. The term oxygen saturation shall be understood to mean the degree of saturation of hemoglobin with oxygen or, more precisely, the ratio between the concentration of the hemoglobin comprising oxygen, known as oxyhemoglobin, and the total concentration of hemoglobin. The oxygen saturation is usually expressed in percent of the maximum saturation and is often referred to as SA or SaO.sub.2.
2. Description of the Prior Art
The process of noninvasive optical measurement of oxygen saturation of blood is based upon the fact, that hemoglobin with bound oxygen, i.e. the oxyhemoglobin, and hemoglobin containing little or no oxygen, known as desoxyhemoglobin, have dissimilar colors and therefore dissimilar absorption spectra. The measurement is generally performed by radiating light of two or more dissimilar wave lengths into a bodily region containing blood vessels and particularly capillary tubes. Then the intensity of light penetrating through said bodily region, such as an ear lobe, the tongue, or a finger, or the luminous intensity of the light reflected back from said bodily region, in particular from the skin thereof, is determined.
Information regarding the measurement principle involved has been known for example from the publication entitled "Noninvasive Transcutaneous Monitoring of Arterial Blood Gases" by Y. Mendelson and R. A. Peura, which appeared in the IEEE Transactions on Biochemical Engineering, Vol. BME-31, No. 12, 1984 on page 792ff. One of the systems for measuring oxygen saturation described in this Publication comprises a measuring head to be set in place in a particular location on the surface of the body. This measuring head is equipped with light emitting diodes adapted to radiate red and infrared light into the respective region of the body, furthermore a photodiode adapted to receive light reflected from the tissue of said region, and a heating device form of a copper ring. During the measuring process the copper ring rests with one of its faces on the skin surface and supplies the skin with heat generated by a heating coil. By applying heat to the region to be checked for its oxygen saturation it is possible to increase the blood supply by as much as twenty to thirty times. This improves the accuracy of measurement, or makes the measurement possible in the first place. A conclusion that may be drawn from the publication under discussion is that an advantageous value for the temperature of the bodily region to be checked for its oxygen saturation is approximately 41.degree..
It is furthermore known, that the skin parties, and particularly the skin cells, as well as the tissue cells lying deeper can get damaged at temperatures higher than 42.degree. C., the survival time of the cells being subject to considerable decrease as the temperature rises above 42.degree. C. The skin covering the body may be subdivided into the outer epidermis and the inner dermis, the latter containing blood vessels. The epidermis, on the other hand, possesses living cells below its lifeless horny layer (stratum corneum) but no blood vessels. When heating the dermis with its blood vessels by the heat it receives from the skin surface by heat conduction, a temperature gradient will arise from the outside toward the inside. The horny layer varies in thickness in different parts of the body, typically between 0.2 and 2 mm, the thickness of the horny layer also depending on the age and the living conditions of the person to be examined. Thus, this known system has the disadvantage, that two possible risky conditions may occur, depending on the value of the temperature generated at the surface of the skin by means of the heating device. In one case, the skin tissue containing the blood vessels may fail to get heated to the temperature required for good measuring conditions, in the second case parts of the skin may become overheated, so that tissue damage may result.
Other known systems are equipped with two dissimilar measuring heads, one of these being provided with light sources and the other with a light receiver. Thus, the first measuring head is adapted to radiate light through a bodily part, the second being adapted to receive this light. These known systems are thus suited for measurement processes in which only light radiated from the examined bodily region back into the measuring head from which it originates, is used, or only light which has penetrated through the bodily part to be checked. Furthermore, the measuring heads of the systems using light to penetrate the bodily part are so built and supported, that they may be used practically exclusively for measurements to be performed on a specific part of the body, such as an ear lobe or a finger. The physician is thus limited to performing his measurements only with light reflected back from the checked bodily region, or only with light penetrating through a bodily part, such as an ear lobe or a finger. As an alternative he would have to buy several different systems adapted for the different types of measurements. Also, in the systems working with light penetrating through the part of the body checked, the light signals reaching the light receivers have light intensities frequently so low, that the results of measurements are quite inaccurate and unreliable.